Currently, a lithium compound (more precisely, a lithium metal oxide) such as a lithium manganese oxide and a lithium cobalt oxide is widely used for a positive electrode active material in a lithium ion battery.
In recent years, more and more researches have been carried out in lithium compounds containing iron, which are used for positive electrode active materials. The lithium compounds containing iron are thermally stable. In comparison to a battery including a widely used cobalt-based lithium compound as a positive electrode material, the stability in battery performance can be further improved with the lithium compounds containing iron. This is a main reason why the lithium compounds containing iron have been focused.
A so-called constant current constant voltage charging procedure is used as a general method for charging a lithium ion battery. In the constant current constant voltage method, a battery is charged with a constant current during an early stage of the charge and the charge is continued while a battery voltage is maintained at a set voltage after the battery voltage has reached the set voltage.
However, a problem occurs when the widely used constant current constant voltage charging procedure is used for a lithium ion battery including a lithium compound containing iron as a positive electrode material.
The present invention was made in view of the foregoing circumstances. An objective of the present invention is to reduce degradation of a lithium ion battery and to provide a method for charging the battery in a short amount of time at low cost.